CHAEACTEES OF AETHEOPODS. 



97 



chain is swollen out into a pair of ganglia united by com- 

 missures in each of the three thoracic segments (fig. 36, d, 3-5), 

 and a pair in the first six abdominal segments. There is also a 

 visceral nervous system (fig. 34, Ve). 



Fig. ; 



-Xrrvous System. 



It, 8uxiiii-CESui)liayeal ganglion; ^, nerve-cnllar ; c, sub-cesniihageal gaDgliou ; 

 d, 3-5, thoracic ganglia ; i.-vii., abdominal ganglia. 



Charaotees of the four Groups of Arthropods. 



The following are the distinctive features of the four divisions 

 of the jointed-limbed animals : — 



1. Crustacea — This is the group that includes the Lob.ster, Crab, Shrimp, 



Prawn, Crayfish, Wood-louse, &c. They are nearly all water-breath- 

 ing Arthropods, respiring by means of brauchire or gills. There are 

 always two paix's of antennEO and compound or facetted eyes. On the 

 abdomen are organs of locomotion. 



2. Arachnoidca — These are the Spiders, Mites, and Scorpions. Respiration 



is various : some respire by lungs, others by tracheae, yet others by the 

 whole body surface. The head and thorax are always united into one 

 piece, the cephalothoras, and in some (Mites) the abdomen is also 

 united to the cephalothoras. Antenns are never formed. There 

 are always four pairs of legs (except in Phytopti), which are never 

 carried by the abdomen. Eyes never compound, ocelli only being 

 present. 



3. Myriapoda — Centipedes and Millipedes. Head quite separate from the 



body, but no distinction between thorax and abdomen visible. One 



G 



